Fixture for Measuring Inductor Q with your Antenna Analyzer

Phil Salas – AD5X

Introduction

Over the past several years I’ve been experimenting with a variety of portable antennas.

These antennas usually use one or more loading coil s in order to keep the antennas

relatively compact. Since my loading coils also tend to by physically small, I have found

it valuable to measure the Q of the antenna loading coils in order to determine the

antenna efficiency.

Determining Inductor Q

At resonance, a tuned circuit has a Q factor equal to the ratio of the inductive or

capacitive reactance to the total series loss resistance in the tuned circuit. If one of the

elements has a significantly higher Q than the other element, then a majority of the loss

resistance can be associated with the lower-Q element. Capacitors generally have a much

higher Q than inductors. And air-variable and mica capacitors typically have a Q well in

excess of 1000. Therefore the loss resistance of a tuned circuit using these types of

capacitors is essentially that of the inductor

Measuring inductor Q is not difficult with an antenna analyzer. First, measure the

resistance of a fixed resistor with the antenna analyzer. Then place the fixed resistor in

series with the inductor to be measured and a high-Q variable capacitor. Tune the

variable capacitor for series resonance at your operating frequency giving the circuit’s

series-resonant resistance which consists primarily of the fixed resistor and the inductor

loss resistance . Subtracting the measured fixed resistor value from the series-resonant

resistance value gives the loss resistance associated with the inductor. Once you know

the inductor loss resistance at resonance, you can determine the coil Q (Q = 2p fL/R). Or

you can use the loss resistance directly to determine antenna efficiency. So, this is easy

to describe. But how do you implement the test set-up? This article describes the

compact and easy to use Q-meter adapter that I came up with.

The Q-Meter Adapter

My Q-meter adapter is shown in Figure 1. I used a 51 ohm metal film reference resistor

since antenna analyzers tend to be pretty accurate as you get closer to 50 ohms. Metal

film resistors are surprisingly non -inductive into the VHF range. I used a MFJ

Enterprises 6-180pf air variable capacitor with a built-in 8:1 vernier drive. Obviously

you can use whatever air-variable capacitor you have on hand, however the vernier drive

in the MFJ capacitor makes tuning very non -critical. I included the capability to switch

in a 180pf fixed capacitor in parallel with the variable should you need additional

capacitance for lower value inductors.

Wiring is not critical over the 1.8-30 MHz frequency range. Everything mounts

comfortably in the small aluminum box called out in the par ts list. The MFJ variable

capacitor has front mounting holes that can easily be tapped for #6 screws. Or you can

use #4 screws/nuts and lockwashers, but mounting these may be a bit tricky. The #4

solder lug mounts under one of the SO-239 screws and is used to ground the 51 ohm

resistor through one switch. The #6 solder lug mounts on the back of the variable

capacitor (tapped #6 hole is already available there) and grounds the mica capacitor

through the second switch. I used slide switches since I prefe r the way they look

compared to toggle switches, but there is much more work involved in cutting out the

mounting holes (I use a small metal file to cut out the rectangular holes). Incidentally,

only SPST switches are necessary, but I liked the size and price of the DPDT switches

called out. The shaft of the variable capacitor is centered on the front of the aluminum

box, and the SO-239 is centered on one side of the box. Both require a 5/8” diameter

hole cut-out. Photo A shows an internal view of the unit. Photo B shows the front panel,

and Photo C shows the Q-meter adapter mounted to a MFJ-259B antenna analyzer with a

double-male UHF connector. All labeling was done using Casio “Black on Clear”

labeling tape.

Using the Q-Meter Adapter

Set the “Additional Capacitance” switch to 0 pf, and the input switch to “50 Ohm

Reference”. Set your antenna analyzer to the desired frequency and record the measured

resistor value. This should be close to 50 ohms with little or no reactance. Next, install

the inductor to be measured across the 5-way binding posts. Flip the input switch to

“Measure Rs” and tune the variable capacitor for resonance as indicated by zero

reactance and a resistance value relatively close to , but higher than, 50 ohms. Add in the

additional 180pf capacitor if necessary for achieving resonance. Record the resonated

series resistance. Now subtract the 50 ohm reference value from the resonated resistance.

The answer is the loss resistance of the coil.

As an example, my favorite portable antenna is a 20 -meter dipole with switches in the

antenna wires that disconnect sections of the antenna to make it resonant from 20-10

meters. For 40- and 30-meters the antenna becomes a center-loaded resonant antenna

using inductors clipped across the 10 meter open switches. Using the Q-meter adapter, I

measured the reference resistor as 51 ohms and the resonant Rs of my 18.5uhy torroid-

wound 40 meter loading coil as 60 ohms. This leaves an inductor loss resistance of 9

ohms, so the coil Q = 2p(7.2)(18.5)/9 = 93. Since the 20-meter antenna is now a 1/4

wavelength center-loaded antenna on 40 meters, some rough calculations indicate that the

antenna radiation resistance should be about 25 ohms. With 9 ohms of loss resistance in

each arm of the dipole, the total loss resistance is 18 ohms for the dipole. Therefore the

efficiency of this antenna on 40 me ters is: Eff(%) = 100 x 25/(25+18) = 58%. Not too

bad, especially for compact loading coils .

My torroid-wound 6.5uhy 30 meter coils only have 3 -ohms of loss resistance , so Q =

2p(10.1)(6.5)/3 = 137. On 30 meters the antenna now becomes a center-loaded 0.36

wavelength dipole w ith an approximate radiation resistance of about 35 ohms. Therefore

this antenna has an efficiency of around 85% (Eff = 100 x 35/(35+6) = 85%). Even

better! You can use equations in the ARRL Antenna Book or antenna modeling software

to determine more accurate values of radiation resistance, but these numbers are probably

pretty close. Of course, this same technique works for determining the efficien cy of

vertical antennas with traps or loading coils as well.

Conclusion

I’ve described a simple , convenient and relatively inexpensive antenna analyzer adapter

that permits you to measure the Q of most inductors used on HF frequencies. Build this

adapter to give you more insight into the efficiency of your inductor designs.

Parts List

Description QTY Source Price ea.

6-180pf Var. Capacitor 1 MFJ 282-5160 $17.97

5-Way Binding Post (Red) 1 MFJ 606-0003 $0.85

5-Way Binding Post (Blk) 1 MFJ 606-0004 $0.85

1-1/2”D Knob 1 MFJ 760-0125 $2.80

DPDT Slide Switch 2 MFJ 501-1003 $0.20

PL-259 Double Male 1 MFJ-7702 $3.95

SO-239 1 MFJ-7721 $1.49

180pf Mica Capacitor 1 Mouser 5982-15-500V180 $0.74

51 ohm ½-watt film resistor 1 Mouser 660-CF1/2C-510J $0.05

AL Box (3.25x2.13x1.63”) 1 Mouser 563-CU-3001A $4.75

#4x3/8” binding head screws 6 Mouser 534-9401 $0.14

#4 nuts 6 Mouser 534-9600 $0.03

#4 solder lug 1 Mouser 534-916 $0.15

#6x1/4” binding head screws 3 Mouser 534-9407 $0.06

#6 solder lug 1 Mouser 534-917 $0.15

Photo A: Internal wiring of the Q-meter Adapter

51 ohm Resistor

180pf SM Capacitor

50 ohm

S2

6-180pf

180pf

S1

REF

Measure

Inductor Under

Test

+180pf

Antenna

Analyzer

Figure 1: Inductor Q Measuring Circuit

Photo B: Q-Adapter front panel Photo C: Measuring inductor Rs